Technical Field
The present disclosure relates generally to driving Field-Effect Transistors (FETs) and, more specifically, to controlling a drive signal applied to an FET to improve the switching operation of the FET.
Description of the Related Art
In converters such as flyback and LLC resonant converters, as well as in other types of converters, when using synchronous rectification in control of the converter, a synchronous rectification circuit drives one or more synchronous rectification (SR) switches during operation of the converter. Each of these SR switches is typically an FET, such as a metal oxide semiconductor FET (MOSFET), although the SR switches may be other types of transistors as well like insulated gate bipolar junction transistors (IGBTs). In the present application, the SR switches are discussed as being power MOSFETs by way of example and those skilled in the art will appreciate that the concepts disclosed herein in relation to such power MOSFETs may be applicable to other types of transistors and circuit structures as well.
A power MOSFET has an associated gate charge curve that defines the relationship between the gate-to-source voltage VGS of the MOSFET and the total charge on the gate of the MOSFET, as will be appreciated by those skilled in the art. The gate charge curve includes several phases or regions, and controlling the current supplied to charge the gate in each of these regions can be advantageous and improve the switching operation of the MOSFET and the converter including the MOSFET. A MOSFET gate charge curve includes two inflection points starting from a zero gate-to-source voltage until the gate-to-source voltage reaches a peak gate drive voltage, which is typically approximately the value of a supply voltage of the circuit driving the gate of the MOSFET. These two inflection points define three regions: 1) a sub threshold region; 2) a saturation or Miller plateau region; and 3) a linear region. In the second or Miller plateau region, for example, the current/charge injected into the gate of the MOSFET does not significantly increase the gate-to-source voltage due at least in part to the gate-to-drain capacitance being substantially charged instead of the gate-to-source capacitance in this region. As a result, the current flowing to charge the gate of the MOSFET in the Miller plateau region may be advantageously controlled to control the length or duration of this region. In this way, the generation of unwanted electromagnetic interference (EMI), which may otherwise be generated by fast rising edges of a gate drive signal applied to the MOSFET, may be reduced by controlling the current supplied by the gate drive signal in this region. During the plateau region the gate-to-source voltage VGS has a slowly changing or approximately constant first gate drive value.
In the linear region, which is the region after the plateau region in the gate charge curve, the gate drive signal typically supplies a large current to quickly charge the gate of the MOSFET and thereby drive the gate-to-source voltage VGS to the peak gate drive voltage. Such a fast rising edge of the gate-to-source voltage VGS may, as previously mentioned, result in the generation of unwanted EMI. In controlling the activation or turning ON of a power MOSFET, as well as deactivating or turning OFF the MOSFET, the current supplied by the gate drive signal to charge or discharge the gate in the respective regions of the gate charge curve is ideally controlled to improve the switching operation of the MOSFET and thereby improve the converter including the MOSFET. There is a need for improved drive circuits and methods for controlling the turning ON and OFF a power MOSFET in converters and other circuits.